The present invention relates to a plasma processing system that supplies plasma producing energy by propagating microwave radiation from the surface of an antenna into a space defined by a processing vessel to produce a plasma.
A plasma processing system is used in a semiconductor device fabricating process for film formation, etching, ashing or the like to deal with the recent progressive device miniaturization and device density increase in semiconductor integrated circuits. Particularly, microwave plasma processing systems capable of producing a high-density plasma by a combination of microwave radiation and a magnetic field created by an annular coil are used prevalently because microwave plasma processing systems are capable of stably producing a plasma in an atmosphere of a comparatively low pressure in the range of 0.1 to several tens mtorr.
A conventional microwave plasma processing system of this kind disclosed in JP-A No. 3-17273 has a plasma producing chamber provided with a magnetic field creating means, and a waveguide connected to the plasma producing chamber to guide microwaves. The microwave plasma processing system produces a dense plasma by electron cyclotron resonance using microwaves.
FIG. 5 is a schematic view of the conventional plasma processing system by way of example. In this plasma processing system, a processing vessel 2 is provided on its upper end with a microwave transmitting window 4. Microwaves of, for example, 2.45 GHz generated by a microwave generator 6 are guided by, for example, a rectangular waveguide 8 to a disk-shaped antenna 10 provided with a plurality of slits. A slow-wave member 12 of a dielectric material, such as a ceramic material, is bonded to the upper surface of the antenna 10 to shorten the wavelength of the microwaves for the improvement of the energy efficiency of the microwave.
The microwaves radiated by the antenna 10 propagate through the microwave transmitting window 4 into the processing vessel 2. Then, a dense plasma is produced in the processing vessel 2 by electron cyclotron resonance (ECR) caused by the microwaves and a magnetic field created by a magnet 14 surrounding an upper portion of the processing vessel 2.
The microwaves propagated through the waveguide 8 and a rectangular/coaxial converter 9 along a coaxial line 16 spread from a central portion of the disk-shaped antenna 10 toward a peripheral portion of the same and are radiated into the processing vessel 2 to supply energy. The energy of the microwaves is supplied into the processing vessel 2 by two energy supplying methods discriminated from each other by the form of the slits of the plane antenna 10.
A first energy supplying method uses an antenna provided with slits arranged at a radial pitch approximately equal to one guide wavelength, i.e., a wavelength determined by the slow-wave member 12, of the microwaves. A second energy supplying method uses an antenna provided with slits arranged at a small radial pitch far shorter than the guide wavelength of the microwaves, such as a pitch in the range of about {fraction (1/20)} to {fraction (1/30)} of the guide wavelength. When the antenna provided with the slits formed at the former radial pitch is used, the microwaves of the same phase propagate downward through the slits as the microwaves spread from a central portion toward a peripheral portion of the antenna, whereby a plasma is produced.
When the antenna provided with the slits formed at the latter radial pitch is used, the microwaves leak little by little through the slits as the same spread from a central portion toward a peripheral portion of the antenna. The microwaves leaked through the slits produce a plasma. The leakage microwaves attenuate exponentially with distance toward a wafer, i.e., toward the bottom of the processing vessel.
A plasma processing system provided with an antenna provided with slits formed at the foregoing small radial pitch is capable of producing and maintaining a plasma by appropriate power (1 to 2 kW for 500 mm in diameter) at a pressure (for example, around 1 mTorr) lower than that required by a plasma processing system provided with an antenna provided with slits formed at the foregoing radial pitch approximately equal to one guide wavelength, without using ECR using an external magnetic field.
However, the antenna provided with the slits formed at the small radial pitch generally is designed to enhance power efficiency by reflecting the microwaves, which is radially propagated from a central portion toward a peripheral portion of the antenna, toward the central portion by the peripheral portion. Thus, there is a tendency for electromagnetic field intensity around the center of the antenna to be higher than that around the peripheral portion of the same.
Consequently, an electromagnetic field intensity on a central region of a surface of a wafer is higher than that on a peripheral region of the same as shown in FIG. 6. Therefore, the plasma is distributed irregularly over the surface of the wafer and hence intra-surface uniform plasma processing of the wafer cannot be achieved.
A system disclosed in JP-A No. 3-224225 discloses a microwave absorber in a waveguide to make uniform the distribution of electromagnetic field intensity of microwaves. This conventional system, however, guides microwaves directly into a discharge tube without using any antenna. Therefore, the technical idea of this conventional system cannot be applied as it is to a system provided with an antenna and the conventional system is unable to achieve the delicate control of the absorption of microwaves.
The present invention has been made in view of the foregoing problems to solve those problems effectively and it is therefore an object of the present invention to provide a plasma processing system capable of making uniform the distribution of electromagnetic field intensity of microwaves by reducing microwaves reflected by a peripheral portion of a plane antenna toward a central portion of the same to some extent.
To solve the foregoing problems, the present invention provides a plasma processing system comprising a processing vessel in which an object to be processed is subjected to a plasma process; a plane antenna for radiating microwaves that attenuate exponentially into the processing vessel; a microwave generator for generating microwaves to be propagated to the plane antenna; a waveguide means for guiding the microwaves generated by the microwave generator to a central portion of the plane antenna; a microwave reflecting member surrounding a peripheral portion of the plane antenna to reflect microwaves, which is propagated from the central portion toward the peripheral portion of the plane antenna, toward the central portion of the plane antenna; and a microwave absorbing means disposed in the peripheral portion of the plane antenna to absorb part of the microwaves that propagate therethrough.
The microwaves propagated from the central portion toward the peripheral portion of the plane antenna and reflected by the microwave reflecting means are absorbed partly by the microwave absorbing means, so that the microwaves are attenuated. Thus, the excessive increase in electromagnetic field intensity of the microwaves in the central portion of the plane antenna is suppressed and, consequently, the electromagnetic field intensity distribution in the processing vessel can be significantly improved.
The microwave absorbing means may include a liquid container disposed in a peripheral portion of the plane antenna; and a microwave absorbing liquid contained in the liquid containers to cause a dielectric loss.
The microwave absorbing means may include a plurality of concentric, annular liquid container disposed in a peripheral portion of the plane antenna; and a microwave absorbing liquid selectively contained in the liquid containers to cause a dielectric loss. The absorbed amount of microwaves can be properly controlled by introducing the absorbing liquid selectively in the liquid containers. Thus, the electromagnetic field intensity distribution in the processing vessel can be further improved.
The plurality of liquid containers may have different radial thicknesses, respectively. The absorbed amount of microwaves can be changed in smaller steps by changing the combination of the liquid containers of different radial thicknesses containing the microwave absorbing liquid to achieve more accurate, fine control of the electromagnetic field intensity distribution.
The plasma processing system may further comprise a circulating means for circulating the microwave absorbing liquid outside the liquid container(s), and a cooling means for cooling the microwave absorbing liquid circulated by the circulating means. The variation of dielectric loss due to the variation of the temperature of the microwave absorbing liquid contained in the liquid container(s) can be suppressed by preventing the temperature of the microwave absorbing liquid from rising. Thus, the uniformity of the electromagnetic field intensity distribution can be further stabilized.